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I have always thought darker colors absorb more heat from the sun, so if you are wearing a white T-shirt you will be cooler under sun than wearing a darker T-shirt, or a black piece of steel will be hotter under the sun than a shiny silvery one.

If this is true, then why is the same not applicable to skin color? It seems to me like the more sun in an area the darker the people's skin has become, isn't that the case?

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Try skeptics.stackexchange.com –  Colin McFaul Apr 28 '13 at 17:39
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Heat is infra red radiation. The more dangerous form of radiation from the Sun, however, is not infrared but ultraviolet. I'm no biologist, but I believe dark skin contains more melanin, which protects against this dangerous UV radiation (UVB anyway). Hence there may be an evolutionary advantage to have dark skin in sunny regions? –  twistor59 Apr 28 '13 at 17:59

4 Answers 4

I agree with what has been said here about melanin and UV light, but I think there is a missing piece to fully understand the color variation in humans. Otherwise everyone would have just black skin and problem solved. One of the counterparts appears to be the conversion of Vitamin D to an active form, that requires the action of sunlight. This trade-off explains the beautiful gradient of color in human populations *.

*Native populations, not migrated populations.

I'm attaching and old but interesting article about this from scientific american 2002. http://www.direct-ms.org/pdf/VitDGenScience/Jablonski%202002%20Skin%20color.pdf

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It is precisely about the balance between vitamin D and UV. Good answer. –  Jeremy Kemball Aug 19 '13 at 14:29

Yes, it is true that the hotter the sun, the darker people's skin becomes (in general) because of the evolutionary advantage conferred by having melanin to protect against UV light. The reason a dark object like an asphalt road gets hot quickly is that colors like black absorb most wavelengths of light, and therefore absorb more energy in the form of thermal energy.

In the same way, the melanin in your skin absorbs the UV from the sunlight because it is so dark, and protects your skin from it.

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It isn't about heat but ultraviolet light. Melanin is the pigment that makes our skin colour whatever it is and in darker skin there's more melanin. Melanin dissipates UV, which otherwise would cause skin cancer as it introduces mutations into DNA. Melanin production is stimulated by UV so that's why tanning beds work, our body senses the danger and responds by making our skin darker. Yes this makes us hotter but that's a small price to pay, melanin needs to be dark to do its job well.

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Here's a map of skin colors of indigenous peoples: 4.bp.blogspot.com/-GhV7vZ-zk_g/TtWrU1rrRUI/AAAAAAAAC88/… Darker pigmentations are at latitudes where UV is highest. Feel free to add the image to your answer if you like. –  kmm Apr 29 '13 at 12:30

I think that you'll need a biologist to answer your question fully. However, I can try to explain what I can in terms of physics. Generally, the darker a material is, the greater the range of wavelengths of light it absorbs. With what is perceived as a perfectly black material by a human reflecting no wavelengths of the visible light spectrum of electromagnetic radiation at all. Therefore it would be logical that in cold climates humans would have darker skin colors in order to absorb a greater range of wavelengths of light. Although at first glance this seems the correct way for humans, and in fact all organisms to evolve, it's obviously not the case. There must be other underlying reasons that have caused organisms to evolve the way they do. An interesting example of how nature seems to defy this is leaves. One would think that the chloroplasts that make up leaves should be black in order to absorb sunlight as efficiently as possible. As we know, this isn't the case.

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In chloroplasts it's simple - chemical reaction needs a photon of certain energy so it could happen. Green photons are useless in this case. –  Juris Apr 28 '13 at 21:02
    
Chloroplasts can't use green light for photosynthesis so they reflect it away hence we see plants as green. There are some photosynthetic organisms that don't use red light. –  AndroidPenguin Apr 29 '13 at 8:02

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